The human brain is a complex machine made up of several important parts that control pretty much all of our every day functions. However, what happens when a part of the brain is damaged or destroyed? In essence, our brain has several different departments that are associated with different functions. The frontal lobe is associated with reasoning, planning, parts of speech, movement, emotions, and problem solving. The parietal love is associated with movement, orientation, recognition, and perception of stimuli. The occipital love is associated with visual processing, and the temporal love is associated with perception, memory, and speech. Clearly, any damage to any part of the brain could completely alter one's ability to "function," in general terms.
There are certain types of brain injuries, but one in particular is noted as the split-brain effect. This is a term that describes the severance of the corpus callosum that connects the two hemispheres of the brain. This can occur when there is damage to the tissue, and often times doctors will perform a procedure called a corpus callosotomy, a procedure used to help alleviate seizures. Why is this important? Let us review some more. The left part of the brain is responsible for speech and contains what is known as Broca's area (speech production) and Wernicke's area (speech comprehension). A person with a split-brain, when shown an image in his or her left visual field won't be able to vocally say or explain what he or she has seen. The person can, however, pick up and show recognition of an object with their left hand, since that hand is controlled by the right side of the brain.
Here is a link to an interesting video that goes into more detail about split-brain procedures and their experiments.